IBM captures first-ever image of single-molecule charge distribution

Enhanced resolution in the Kelvin probe force microscopy images by tip functionalization with a carbon monoxide molecule. When a scanning probe tip is placed above a conductive sample, an electric field is generated due to the different electrical potentials of the tip and the sample. With Kelvin force probe microscopy this potential difference can be measured by applying a voltage such that the electric field is compensated. Therefore, KPFM does not measure the electric charge in the molecule directly, but rather the electric field generated by this charge. The field is stronger above areas of the molecule that are charged, leading to a greater KPFM signal. Furthermore, oppositely charged areas yield a different contrast because the direction of the electric field is reversed. This leads to the light and dark areas in the micrograph. Image courtesy of IBM Research - Zurich

IBM
scientists were recently able to measure for the first time how charge
is distributed within a single molecule. This breakthrough should enable
fundamental scientific insights into single-molecule switching and bond
formation between atoms and molecules. The ability to image the charge
distribution within functional molecular structures holds great promise
for future applications such as solar photoconversion, energy storage,
or molecular scale computing devices.

As reported recently in the journal Nature Nanotechnology,
scientists Fabian Mohn, Leo Gross, Nikolaj Moll and Gerhard Meyer of
IBM Research succeeded in imaging the charge distribution within a
single molecule by using a special kind of atomic force microscopy
called Kelvin probe force microscopy at low temperatures and in
ultrahigh vacuum.

"This
work demonstrates an important new capability of being able to directly
measure how charge arranges itself within an individual molecule,"
states Michael Crommie, professor in the Department of Physics at the
University of California, Berkeley. "Understanding this kind of charge
distribution is critical for understanding how molecules work in
different environments. I expect this technique to have an especially
important future impact on the many areas where physics, chemistry, and
biology intersect."

The
new technique provides complementary information about the molecule,
showing different properties of interest. This is reminiscent of medical
imaging techniques such as X-ray, MRI, or ultrasonography, which yield
complementary information about a person's anatomy and health condition.

The
discovery could be used to study charge separation and charge transport
in so-called charge-transfer complexes. These consist of two or more
molecules and hold tremendous promise for applications such as
computing, energy storage or photovoltaics. In particular, the
technique could contribute to the design of molecular-sized transistors
that enable more energy efficient computing devices ranging from sensors
to mobile phones to supercomputers.

For their experiments the IBM scientists used their home-built combined scanning tunneling microscope (STM) and atomic force microscope (AFM). In this focused ion beam micrograph, the tip attached to a tuning fork can be seen. The tuning fork measures a few millimeters in length. The tiny tip measures only a single atom or molecule at its apex. Image courtesy of IBM Research - Zurich

"This
technique provides another channel of information that will further our
understanding of nanoscale physics. It will now be possible to
investigate at the single-molecule level how charge is redistributed
when individual chemical bonds are formed between atoms and molecules on
surfaces," explains Fabian Mohn of the Physics of Nanoscale Systems
group at IBM Research – Zurich. "This is essential as we seek to build
atomic and molecular scale devices."

Gerhard
Meyer, a senior IBM scientist who leads the scanning tunneling
microscopy (STM) and atomic force microscopy (AFM) research activities
at IBM Research – Zurich adds, "The present work marks an important step
in our long term effort on controlling and exploring molecular systems
at the atomic scale with scanning probe microscopy."

For
his outstanding work in the field, Meyer recently received a European
Research Council Advanced Grant. These prestigious grants support "the
very best researchers working at the frontiers of knowledge" in Europe.

Taking a closer look

To measure the charge distribution, IBM scientists used an offspring of AFM called Kelvin probe force microscopy (KPFM).

When
a scanning probe tip is placed above a conductive sample, an electric
field is generated due to the different electrical potentials of the tip
and the sample. With KPFM this potential difference can be measured by
applying a voltage such that the electric field is compensated.
Therefore, KPFM does not measure the electric charge in the molecule
directly, but rather the electric field generated by this charge. The
field is stronger above areas of the molecule that are charged, leading
to a greater KPFM signal. Furthermore, oppositely charged areas yield a
different contrast because the direction of the electric field is
reversed. This leads to the light and dark areas in the micrograph (or
red and blue areas in colored ones).

Schematic of the measurement principle. At each tip position, the frequency shift is recorded as a function of the sample bias voltage (inset, red circles). The maximum of the fitted parabola (inset, solid black line) yields the KPFM signal V* for that position. Image courtesy of IBM Research - Zurich

Naphthalocyanine,
a cross-shaped symmetric organic molecule which was also used in IBM's
single-molecule logic switch, was found to be an ideal candidate for
this study. It features two hydrogen atoms opposing each other in the
center of a molecule measuring only two nanometers in size. The hydrogen
atoms can be switched controllably between two different configurations
by applying a voltage pulse. This so-called tautomerization affects the
charge distribution in the molecule, which redistributes itself between
opposing legs of the molecules as the hydrogen atoms switch their
locations.

Using
KPFM, the scientists managed to image the different charge
distributions for the two states. To achieve submolecular resolution, a
high degree of thermal and mechanical stability and atomic precision of
the instrument was required over the course of the experiment, which
lasted several days.

Moreover,
adding just a single carbon monoxide molecule to the apex of the tip
enhanced the resolution greatly. In 2009, the team has already shown
that this modification of the tip allowed them to resolve the chemical
structures of molecules with AFM. The present experimental findings were
corroborated by first-principle density functional theory calculations
done by Fabian Mohn together with Nikolaj Moll of the Computational
Sciences group at IBM Research – Zurich.